The present invention generally relates to a method of preparing precoated press-hardened steel, and more particularly to pre-diffusing or pre-alloying the coating with the iron-based substrate to enable high rate heating of the blank immediately prior to hot press forming.
Steel and related structural materials used in automobile manufacture are increasingly required to simultaneously exhibit reduced weight and enhanced crash-worthiness features. One way to produce steel capable of maximizing these hitherto conflicting goals is to use high strength press-hardened steel, where component forming and hardening operations take place within a single step. Such an approach can lead to desirable properties, such as providing structural steel parts with significant increases in strength-to-weight ratio. In press-hardening, steel strip, roll, cut pieces, blanks or related workpieces are heated to austenite temperature and then formed into a final (or near-final) shape while simultaneously being cooled into the final martensitic microstructure. Current heating methods for use with press-hardened steel include using either tunnel-style (radiant tube) furnaces or vertical box-type (electric or radiant tube) furnaces.
In one form, the steel workpiece may be pre-coated, where the coatings, such as aluminum-based ones, can be used to provide a protective layer to the underlying steel workpiece. The use of such coatings enables a simpler manufacturing process, as inert furnace atmospheres and post-forming cleaning operations may no longer be required since scale formation is eliminated. Additionally, such coatings improve barrier corrosion performance of the underlying iron-based workpiece. One particular form of such a coating is aluminum-silicon alloy (Al—Si) that, when placed on the iron-based substrate and subjected to elevated temperatures, allows the diffusion of the iron from the substrate into the coating.
Unfortunately, the slow heating rates employed during the austenitization step in traditional press hardening requires extensive furnace capacity and significant manufacturing floor space. Additionally, the ability to rapidly heat the steel blanks to relatively high temperatures (typically in excess of 880° C.) for use in press hardening has been deemed incompatible with the preferred slow heating rates of the low melting point of the coatings (where, for example, it is about 660° C. for pure aluminum or around 577° C. at the Al—Si eutectic) that are used to promote the iron diffusion into the coating as a way to avoid detrimental localized melting of the coating. Likewise, high heating rates during the blank austenitization step in press hardening needed for high-volume automotive production and related high strength-to-weight components would destroy the very coating used to provide protection to the iron-based substrate.